1-Piece Upper Receiver with Back Up Iron Sights (BUIS)
Full Floated Barrel
Bayonet Lug and Flash Hider
Accepts Mil-Spec 5.56 Colt Uppers
I would say it accepts the normal mil spec AR upper of most any brand .
This pic explains more how it works than any I have found , it uses the adapter to make the smaller upper work which also makes use for your smaller 5.56 mags